ABSTRACT The striatum is the primary input structure of the basal ganglia. While the striatum has been associated with many neuropsychiatric and neurological disorders, the circuit mechanisms underlying these disorders are largely unknown. Several of these disorders, including obsessive-compulsive disorder (OCD), Tourette's syndrome, and dystonia, have been linked to defects in fast-spiking interneurons (FSIs), a class of GABAeric interneurons in the striatum. My preliminary data established a connection between striatal FSIs, feed-forward inhibitory microcircuits, and motor-sequence learning that is relevant to movement disorders. Through the proposed research, I will investigate the mechanisms by which the striatal inhibitory microcircuitry contributes to neuropsychiatric disease and cognitive flexibility. Ragozzino and colleagues have shown that acetylcholine levels in the striatum are elevated during reversal learning tests of cognitive flexibility, a basal ganglia-related form of learning that is impaired in neuropsychiatric disorders. They also showed that task performance relies on muscarinic acetylcholine receptor (mAChR) signaling in the striatum. My preliminary results show that striatal mAChR signaling predominantly suppresses FSI-mediated feed-forward inhibition, supporting the idea that modulation of FSIs is critical for task performance. Indeed, striatal FSIs are active when rodents make choices, especially in decision-making under conflict, and are therefore likely involved in cognitive flexibility. Thus, I will combine slice physiology, behavior, and in vivo recordings to test the hypothesis that activation of mAChRs suppresses FSI-mediated feed-forward inhibition to increase the variability of striatal network responses for reversal learning and cognitive flexibility. With slice physiology, I will investigate the microcircuit, neuromodulatory, and physiological mechanisms linking feed-forward inhibition to variability in network responses. With a novel reversal-learning task that I developed, I will then test the importance of mAChR signaling and striatal FSIs in cognitive flexibility. Then, with optogenetic manipulation and in vivo physiology, I will answer the controversial question of whether FSIs inhibit medium spiny neurons (MSNs) in vivo. I will directly test how this circuit affects the variability of network responses in vivo by manipulating FSIs and cholinergic signaling while recording striatal network activity during reversal learning. These experiments will expand my expertise in behavioral assays of cognitive flexibility and analysis of in vivo physiology data. I have assembled an outstanding Advisory Council that includes world experts in basal ganglia physiology, dissection of inhibitory microcircuitry, and neuromodulation. Two psychiatrists on this team will provide guidance on disease relevance of the experimental design and results. The proposed work will allow me to gain the necessary skills and preliminary data to establish and run a productive independent research program and successfully compete for R01 funding to study striatal microcircuitry in cognitive flexibility and neuropsychiatric disease.